Embodiments of the present disclosure relate to wind turbines, and more particularly to reducing tower oscillations in wind turbines.
Wind turbines are increasingly gaining importance as renewable sources of energy generation. In recent times, wind turbine technology has increasingly been applied to large-scale power generation applications. A wind turbine typically includes a tower and a rotor rotatably coupled to two or more blades. Maximizing energy output while minimizing loads of the wind turbines in varied wind conditions is a challenge that exists in harnessing wind energy.
Tower oscillations or vibrations may cause significant loading of a wind turbine and may result from various disturbances such as turbulence, large and sudden gusts, inefficient damping, or transitions between wind conditions. A tower may vibrate along any degree of freedom. For example, the tower may vibrate in a fore-aft direction (commonly referred to as tower nodding), in a side-to-side direction (commonly referred to as tower naying), or along its longitudinal axis (commonly referred to as torsional vibration).
Tower nodding is usually caused by aerodynamic thrust and rotation of the blades. Every time a rotor blade passes in front of the tower, the thrust of the wind impinging on the tower decreases. Such continuous variation in wind force tends to induce oscillations in the tower. Moreover, if the rotor velocity is such that a rotor blade passes over the tower each time the tower is in one of its extreme positions (forward or backward), the tower oscillations may be amplified. Oscillations in the fore-aft direction are sometimes “automatically” minimized due to aerodynamic damping which relies on the fact that the top of the tower constantly oscillates in the fore-aft direction. When the top of the tower moves upwind (or forward), the rotor thrust is increased. This increase in rotor thrust pushes the tower back downwind. The downwind push in turn aids in dampening the tower oscillations. Similarly, when the top of the tower moves downwind, the rotor thrust may be decreased. This decrease in rotor thrust pushes the tower back upwind. The upwind push also aids in dampening the tower oscillations.
Although aerodynamic damping aids in reducing oscillations under many circumstances, if the rotor velocity is synchronized with the tower oscillations, the tower may oscillate at a high rate causing mechanical strain and possible damage to the tower. Moreover, such synchronization may amplify the rotor velocity at tower resonance frequency, thereby potentially damaging generators and/or drivetrains connected to the rotor blades. Even when the aerodynamic damping aides in reducing oscillations, the damping is a reactive technique that begins only after tower vibrations and oscillations have occurred. Therefore, a tower relying on this type of damping needs to be sturdy enough to sustain loads till aerodynamic damping techniques are activated. For reducing the cost of energy, it is expected that different types of towers and blades will be utilized. To enable flexibility in design, extreme loads needs be predicted and prevented.
Therefore, there is a need for an enhanced method and system for preventing occurrence of extreme loads in wind turbines.